For the brain, as compared to most other organs, e.g., the liver, there are major limitations involved in examining a single neuronal cell type in isolation. Brain developmental and functional processes are highly dependent on interactions between different neural cell types and between different regions of the brain (for example, De Marco Garcia et al., Nature 472:351-355, 2011; Nishi, Prog. Neurobiol. 69:213-227, 2003). A great deal of attention has, for example, been focused on differentiation of human mesencephalic dopaminergic (mDA) neurons from human pluripotent stem cells (hPSCs) (Kriks et al., Nature 480:547-551, 2011; Perrier et al., Proc. Natl. Acad. Sci. USA 101:12543-12548, 2004; Vazin et al., PLoS One 4:e6606, 2009; Yan et al., Stem Cells 23:781-790, 2005; Zeng et al., Stem Cells 22:925-940, 2004) because of their potential for use in transplantation therapy for Parkinson's disease. The development and formation of mDA neurons does not, however, occur in isolation, and dopaminergic (DA) systems and their target structures are highly interdependent (Halliday et al., Brain Res. Mol. Brain Res. 85:171-178, 2000; Hemmendinger et al., Proc. Natl. Acad. Sci. USA 2:1264-1268, 1981; Hoffman et al., Brain Res. 274:275-281, 1983; Parish et al., J. Neurosci. 21:5147-5157, 2001; Prasad and Pasterkamp, Adv. Exp. Med. Biol. 651: 91-100, 2009; Shalaby et al., Brain Res. 307:347-350 1984).
Dopaminergic neurons play an important role in substance abuse (Volkow et al., Mol. Psychiatry 9:557-569, 2004; Wise, Biol. Psychiatry 73:819-826, 2013), motivational processes in general (Wise, Nat. Rev. Neurosci. 5:483-493, 2004), and in the pharmacotherapy of schizophrenia (Knable and Weinberger, J. Psychopharmacol. 11:123-131, 1997; Weinberger and Lipska, Schizophren. Res. 16:87-100, 1995). Both schizophrenia and substance abuse are believed to involve interactions between mDA neurons and target cells in the forebrain (Knable and Weinberger, J. Psychopharmacol. 11:123-131, 1997; Koob and Volkow, Neuropsychopharmacol. 35:217-238, 2010; Weinberger and Lipska, Schizophren. Res. 16:87-100, 1995). Therefore, the possibilities for employing either mDA neurons or neocortical neurons derived from hPSCs in isolation to elucidate substance abuse or schizophrenia are very limited.